Hydraulic brake system and pressure regulating device therefor



April 27, 1965 w. .srl-:LzER

HYDRAULIC BRAKE SYSTEM AND PRESS URE REGULATING DEVICE THEREFOR Filed Sept. ll, 1963 URNNMW NA KUNE.

s f7 b mvENToR Vl/z//zdm 5757267 `This invention relates to hydraulic brake systems for vehicles, and particularlyto a pressure transmitting device yoperable-to `deliver brakefluid under varying pressures vand 1n differing volumes from themaster cylinder to the front and rear wheel brake cylinders in a vehicular hydraulic bnake system.

In the brake industry it is well known that the maximum .braking effort of which the front andV rear Wheels of a United States Patent i ice vehicle -are capable without Vskidding varies with the rate of deceleration ofthe vehicle. This is because of the fact that, during braking, a couple is produced about the center ofgmass of the vehicle lwhich transfers a portion of the i' yweight of the vehicle from the rear wheels to the front Wheels. The braking effort of which a wheel is capable borne by such wheel. Accordingly, as the rate of deceleration is increased, a' Agreater percentage of the braking force may be applied through .the front wheels without producing skidding.y

Conventionally, the percentage of braking effort sought to be derived from the front and rear wheel brakes is static, this ratio generally being controlled by the relative sizes of the front and rear wheel brake cylinders. In

such brake systems, if the rear brake cylinders aremade very small compared to the front, then, excessive wear results on the front brake linings andthe rear brakes do not deliver the full braking effort of which they would otherwise be capable at normallow rate of deceleration stops. On the other hand, if the front brake cylinders `are not made large enough .the vehicle is subject to pref mature rear wheel skidding during full power high-speeld braking. Under such conditions, it requires a longer distance in vwhich to stop the vehicle and control of the vehicle is easily lost. l

ln addition to the problems produced by weight transfer, modernbraking systems may use different types of ,brakes on the front and rear wheels which have differing draulic pressure toovercome the force of lthe retraction Ysprings and merely bring the brake shoes into contact with -the kbraking surface of the drum. Incontrast, disc `brakes vare effective to deliver some bnaking elort at a very low pressure inasmuch as any retraction springs which Aare rused in disc brakes are generally ofra very low strength.V

Ithas also been found that disc brakes are subject to what ,has'been termed pad knockoff. This expression refers to the .displacement of the disc brake linings away from the normal planes of the braking surfaces of the disc as a result of the deflection of the disc produced by bumps or Lthe like on the driving surface. Under such circumstances,

a large volume of uid is required to move the disc pads varies directly with the percentage of the vehicle Weight ananas n l'atentecl Agen-27, i965 or linings into contact with the disc prior to the application of any actual braking load to thewhe'els It is, of course, desirable to accomplish this lining movement without excessive pedal travel. Y

Workers in the brake eld have heretofore proposed various devices to compensate for the weight transfer between the rear and front wheels. However, such devices have almost always involved some loss in the total braking yeffort produced for a given pedal effort. Furthermore, such devices have made no provision for initially diifering volume and pressure requirements of rfront and rear,

brakes. Accordingly, it is an object of the present invention to provide a pressuretransmitting device for hydraulic brakes which is operable both to modulate the pressure delivered to the frontr and rear brake cylinders substantially in accordance with the dynamic weight transfer and which is alsooperable to compensate for the varying volume and pressure requirements of front and rear brakes to bring the linings of such brakes rapidly into contact with their associatedbraking surfaces.

It is another object of the present invention to provide a pressure transmitting brake of the above character in which the ratio of pedal effort to hydraulic pressure delivered tothe brakes during a hightdeceleration stop is relatively low.

It is another object of the present invention to provide a rpressure transmitting device of the above character which is operable to stop the vehicle on which it is used in a relatively short distance without premature rear wheel skidding and without excessive bnake pedal travel.

It is another object of the present invention to provide a pressure transmitting device of the above character which eliminates the necessity for the conventional residual valve employed in master cylinders of hydraulic brake systems.

It is still another object of the present invention to provide a pressure transmitting device of the above chanacter in which increasing master cylinder pressure produces a smooth gradually increasing level of wheel cylinder pressure without sudden rapid increases in wheel cylinder pressure or undesired periods of constant wheel cylinder pressure, thereby avoiding any abrupt or sudden application of braking force lat the front or rear wheels.

it. is another object of the present invention to provide a pressure transmitting device of the above character which is reliable inyoperation; which is not subject to external leakage of brake fluid; which is sturdy in construction and which may be manufactured at a relatively low initial cost.

These and'other objects of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a longitudinal sectional viewof the pressure transmitting device of the present invention shown in association with a diagrammatically represented brake system, and

FIGURE 2 is a graph showing the front and rear wheel brake pressures as a function of the master cylinder brake pressure in a brake system employing the device of the -present invention.

essonne Y brake .iiuid under pressure strom a conduit .lo connected tothe outlet of the master cylinder '14 and to deliver brake uid through a conduit i3 to `the bralre cylinders Zllof the rear Wheels 22 of the vehicle; The device i@ also delivers uid under pressure through a conduit 24 .tor the brake cylinders 2b of the front Wheels 28 of the vehicle. A return conduit 30 leads from the device l@ to the brake uid reservoir 32 of the 'master cylinder le.

The proportioning devicelll Vincludes a housing which may be Conveniently formed in two housing portions 3d and 36 Secured together by any suitable fastening, such as, EaP Smell/ d, and with a suitable seal, such as an bung 4%, `interposed between the mating faces thereof; ghe housing .3436 is closed at its opposite ends and is formed with coaxial bores of varying diameter, each jbore defining a piston and tluid receiving chamber. The housing portion 35 has a bore defining a chamber 42, lWhile the housing portion 34 has a plurality of bores delining chambers '45s, '46 and 48, each of successively de- `creasing di-anieter. The chambers 42, .44, la and 4S are interconnected and are coaxially arranged. The charnher 2id is of greater diameter than the chamber 42, While fthe chamber 46 and 48 are each smaller than the charnjber 42. The piston Sti is operably disposed in the chamiberZ and has an annular unidirectionally acting seal 52 slidably sealingly contacting the Wall of said chamber. This piston 59 is formed with an axial bore dening'a chanab'er 53 open to the left-hand endthereof.` An extension 54 of a piston Se is fitted in the chamber 53 to4 join the .piston 56 to the piston 5d. The piston 56 will be seen to have duid displacing piston portions 5S, et? and 62 adapted to operate Within the chambers ltd, 46 and 48 respectively. The piston portion 58 is provided with an annular unidirectionally acting seal ed'sealingly contacting the wall of the chamber ed, the piston portion 6? is provided with an annular unidirectionally acting seal do sealingly and slidably engageabl'e with the Wall of the chamber d6, and the piston portion d?. is provided with an O-ring seal 63 sealingly engaging the Wall of the chamber 4S. The piston 56 has a centrally located axially v extending through passage 7b communicating with a pas- 'sage i2 extending throughout the length of a'piston 73 'operably disposed in the chamber 53. The passage '72, in turn, communicates with the chamber 42 through ports di)y and 92 formed in the pistons 5t? and '73, respectively.

From the foregoing, it will be seen that the pistons 5l) y fand S6 are joined for movement together and may be :considered as one member. Upon the pressurization of the right-hand end of the chamber 42 the pistons 50 and y'Sti move to the left (as viewedin the drawing) to displace uid to the .front Wheel brake cylinders 26 and pressurize said cylinders. The rear Wheel brake cylinders, however, are initially pressurized by the direct flow of fluid from the chamber t2 to the vchamber 48 through the passages 7d and '72. By tins means a pressure is developed in the chamber 4S (and, thus, in the rear Wheel brakek cylinders 2lb) equal to the master cylinder brake pressure present at the right-hand end of the chamber 42. Therefore, the passages 7@ and 72 may be characterized as by-pass meansfor the direct ilow'of iluid to the rear brakes past` the pistons, v :The eiect Whichthis arrangement has upon the initial volume and pressure of fluid delivery to the brake cylinders Will be subsequently considered.

The pressurization of the brake cylinders in the foregoing manner prevails from the beginning of brake application until a preselected master cylinder pressure has been reached, At this point a change in the arrangement for pressurizing the brake cylinders is made and this change is activated by the closure of the by-pass means deiined by the passages 7@ and '72. A valve element 76 for closing the by-pass means is located Within a counter.. borel'd .formed in theright-hand-end of Lthe passage 72.

`Avalve element '7 6 is adapted to seal againstan end vvall 78 of the counter bore '74 and closeV oif the flow Giluid through the passage 72. The valve .element 7e has a valve stern Si? connected at its, lefthand end to' apertured plater4 almlttin"Y a split lock ring Se carried by the piston Si?. A positioning spring 8o, seated against lthe right-hand end ofthe chamber 53, holds plate SZ against the ring 84 and, thus,v maintains thetvalve element '76 in a fixed position with lrespect to the piston Sil.

Brake iiuid under pressure ows into the chamber d?, through. a housing inlet openingi connected to the conduit 16. 4 From thecliarhber 42, master` cylinder fluid pressure has access to the left-hand end of they piston '73 through the ports 9i) and 92 andthe passage '72,'.Y During `initial braking the piston 73 is biased against the plate 82 `by a spring'96v/hich is seated against a, radial shoulder 98 formed in y,the piston 56 atfone end of the chamber 53. The spring' 9o is of a selected strength to yield when the lluid pressure acting against the lett-hand end of .the

` piston 73 is of a given'vaiue. When the springl yields',

iiuid pressure nives the piston 'i3 to the' right causing the valve element '76 to seal against the wall 7S and close off the passages 7b and 72 rommastercylinder pressure. The chamber 53 and `the piston '73 are both 'of stepped ,diameter providing an annular space occupied by the spring $6. This space is vented through a port lil@ in the piston 'Sil tolan annular clearance space in the chambers 42 and ddrbetween the seals 52` and This space is, in'turn, vented through a port H32 in the housing portion 34 to a chamber ltl4,also formed inthe housing portionl 34. f The chamber i434 is infcornmunication with housing opening 1% connected tothe conduit 3b, leading tothe tiuid reservoir By this means the an; nular space 'occupied bythe springV and the annular space between the seals 52 and 64 are maintained at atmospheric pressure.

Prior to brake operationthe pistons Si? and 56 are maintained in the positions illustrated, with the piston 5% caring against .the end Wall of the housing portion 36. This is accomplished'Y byrneans of a spring ltlS which. bears at its one kend against an annular shoulder 1li) of the housing portion Sda'nd its lother end against an annular radially outwardly extending ilange 112 formed on the piston portion 56 to the lett of the seal 64.1. As .iluid pressure is generatedwithin the chamber 42 it acts against the right-hand side of the piston 5o and moves the pistons 59 and 56 to the'lett against the force of the spring 16S. In so doing the'piston portions 5S and o@ will displacetluid through a housing passage 'M4 leading to an outlet opening 116 in the housing portion 3d connected to the conduit 24 and the front Wheel cylinders 26. Fluid in the chamber 4d is displaced tothe chamber' 46 past the unidirectionally` acting seal 66 when theiluid in the Y chamber 44 is at a higher pressure. It will be noted that the bore defining the chamber 46 has a plurality of axially extending groovesy liti adjacentthe right-hand end thereof, which initially provide a bypass past the seal 66. However, the grooves 11S` are'` merely for the relief ofV pressure in the chamber 46 upon thev release of the brakes,V and vfluid is still free to pass from the chamber 44 to the chamber 46; even when theseal 66 is out ot registry with the grooves 118.

The spring M58 is a Vlight-Weight return spring Vand While it oliers some resistance to piston movement, this may be allowed vtor in the design. Accordingly, the pressure of the huid displaced` to the front Wheel cylinders will bear a denite known relationship to master cylinder pressure. Disregarding the spring 168, the ratio of thetvvo pressures is equal to the inverseA ratio of the effective piston areainvolved. The effective area of the piston portions 58 and ell pressurizing fluid to Ythe front wheel cylinders 25 is equal to the-cross-sectional area of the chamber ed less the cross-sectional area of the chamber 43. The etlective areaof the piston Si) subject to the biasing force of master cylinder pressurev equals v of the chamber 53 at the right-hand end thereof.

the cross-sectional yarea of the chamber 42 less the crosssectional area oi the chamber 48 (it being apparent that luid in the chamber 48 opposes the force of luid in the chamber 42 moving the piston 50 to the lett).v The combined effective areas of the piston portions 58 and 60 vbeing greater than the eiective area of the piston i) during initial Ioperation of the device, the pressure of fluid delivered to the front wheel cylinders will be less than master cylinder pressure and, thus, less than the pressure delivered to the rear wheel brake cylinders 26. Furthermore, the lar-ge areas of the piston portions 58 and 60 assure a large volume uid flow to the front brakes to compensate for any greater volume requirement 'of the front brakes, such as is occasioned by pad knock ot.

When the pressure of the fluid in the chamber 42 produced by the master cylinder 14 has reached a selected level, the by-pass provided by the passage 70 and 72 is closed to initiate the aforementioned gradual change in the ratio of front and rear brake cylinder pressures. rEhe closure of said by-pass by the valve element 76 results from the yielding of the spring 96. The spring 96 initially maintains the piston 73 in the illustrated positions. However, the piston 73 is subjected to a force from tluid pressure in the left-hand end of the chamber 53 of the piston Si) attempting to compress the spring 96. The piston 73 has a large diameter end sealingly slidable in the chamber 53 and also a small diameter end sealingly slidable in a small bore 119 which forms an extension Master cylinder pressure acts against the small diameter end of the piston 73. Accordingly the effective area ofthe piston 73 subject to iiuid pressure -for movement against the spring 96 equals the cross-sectional area of the chamber 53 less the cross-sectional area of the bore 119. The strength of the spring 96 is selected so that the spring will yield at the desired master cylinder pressure, as for example, 150 p.s.`i. When the spring yields the piston '73 moves to cause its wall 78 to seal against the valve element 76, thereby closing the by-pass.

When the valve 76 closes, the forces produced by uid pressure acting on the opposite sides of the piston assembly Sil-56 are in balance. The total force produced by the liuid in chamber 42 acting against the piston 510 over the whole cross-sectional area of the chamber 42 equals the :forces produced by uid pressure in the chambers 44, 46 and 4S actingagainst the eiective areas of the piston portions 58, 60 and 62, respectively.v After closure of the valve 76, the relative proportion of the reactive force attributable to each of the piston portions 58,76) and 62 undergoes a gradual but marked change. The result of this change may be seen by reference to the exemplary chart of FIGURE 2 wherein it is assumed that the valve 76 closes at 150 p.s.i. master cylinder pressure. Thereafter, it will be seen that the proportion of pressure at the front brake cylinders increases and the proportion of pressure at rear brake cylinders decreases. This is desirable in order to cause the braking eiort of the front and rear wheel brakes to closely approximate the proportion of the lweight of the vehicle borne by such wheels; In this connection, it is assumed that during high energy stops the weight transfer will vary in accordance with master cylinder pressure. While this is an'approximation only, tests -have indicated that the relaitionship is sufiiciently close that highly desirable results may be obtained when using master cylinder pressure as a measure o weight transfer. From the foregoing it will be apparent that the ratio of the pressure in the chamber 46 compared to the pressure in the chamber 48 increases with increasing master cylinder pressure.

t will be apparent that with the closure of the bypass -further pressurization of front and rear wheel cylinders is solely dependent upon the fluid displaced to them yby the piston portions 58, 60 and 62. Also, in the absence of other inuences, the relative pressures in the 6 -front and rear wheel brake cylinders will depend on the relative areas of the piston portions displacing fluid to them. )In the illustrated embodiment of the invention the various bore shaped chambers of the housing 341-36 may, for exemplary purposes, be considered to have the `following cross-sectional areas: Chamber 4t2- 1.5 sq. inches, chamber df- 2.4 sq. inches, chamber i6-1.2 sq. inches and chamber Atti-.3 sq. inch. The eiective areas of the piston portions 58 and 60 are, therefore, 2.1 sq. inches and .9 sq. inch, respectively. It will, thus, be seen that the piston portion 66 is, by itself, effective to deliver a greater volume of iluid to the yfront brakes than the piston portion 62 is capable of displacing to the rear brakes. The primary consideration, however, is that after closure of the by-pass, the volumes of i'luid displaced to the front and rear brakes are in a fixed ratio.

The device of the present invention Vis designed so that after closure of the valve 76 displacement of iluid lto the front wheel brake cylinders is accomplished solely by the piston portion 60. This results from the provision of fluid venting means for the chamber. 44. This means includes a housing passage leading from the chamber 44 to a passage 122 in communication with the chamber 11M. This provides a path .for the iiow of brake uid from the chamber 44 to the master cylinder reservoir, which is always at atmospheric pressure. During initial brake application this path is closed by a ball valve 126 sealing against an annular conical valve seat 124 at one end of the chamber 164 and surrounding one end of the passage 122. The ball valve 126 is normally kept closed by a spring 12S. The spring 128 is designed to yield at the same time as the spring 96, operatively associated with valve element '76. The spring 128 is initially overcome solely by the force exerted against the ball valve 126 by fluid in the passage 122, which is at the same pressure as iluid in the chamber 44. By this means the further buildup of pressure in the chamber 44 after closure of the valve 76 is arrested.

The elimination of the piston portion 58 as a brake cylinder pressurizing media reduces the overall area of the piston assembly reacting against brake :fluid in opposition to the applying force of master cylinder pressure against the piston 5i). In the illustrated example, the piston portions 60 and 62 have eiective areas of '.9 sq. inch and .3 sq. inch for a total of 1.2 sq. inches which is less than the gross area of the piston 5i?. Thus a gain in the total pressure delivered to the brake system results from the use of the device and a smaller power booster may be used where it is desired to employ a power booster in a brake system in which the device of the present invention is used. Nevertheless, the additional piston portion 5S does give added volume of fluid displacement during initial brake application to compensate for initial high volume requirements of front brakes.

It will be apparent that the presence of iluid under pressure in the chamber 44 after closure of the valve element 76 constitutes a needless resistance to the movement of the piston assembly Sil-56,. If this pressure were immediately released in its entirety, front wheel brake pressure lwould undergo a very sudden and irregular increase, and a simultaneous irregular change in rear wheel brake pressure would result. VIn order to produce a gradual bleeding oil of fluid pressure in the chamber 44 and produce the desirable gradual modulation of brake pressure depicted in FIG. 2, a plunger 129 is reciprocally positioned in the passage 122 with a head portion 138 disposed in the passage 114 for exposure to front brake cylinder pressure. The plunger 129 has a reduced diameter stem 136 engageable with the ball valve 12,6. Fluid is free to ilow around the stem 130 in the passage 122 between the chamber 44 and the ball valve 126. At the time of closure of the valve 76, the pressures acting against the opposite ends of the plunger 129 are equal. Accordingly, the'plunger 129 applies no force to the ball valve 126. However, the `front brake cylinder pressure acting against the plunger head S138 rises as master cylinder pressure rises, while the pressure in chamber d tends to remain static. Therefore, the plunger stem i3@ is biased against the ball valve 126. This reduces the pressure in chamber del needed to open the ball valve 26 and bleed ntl lluid from the chamber 4d', The biasing force of the plunger 129 against the ball Valve E26' increases as master cylinder pressure increases until eventually the pressure in chamber dd is reduced to atmospheric pressure.

leverting brieily to the unidirectionally acting seal 66, it will be seen that this seal is held against a radial piston shoulder 146 by a spring ldd and is provided with a thin ilexible lip around its periphery. The lip 15d is inclined outwardly and forwardly. Where a greater pressure exists ahead of than behind the seal, tluid pressure forces the lip against the wall of the chamber sld. Where the.L 'reverse pressure con lition prevails, however, the lip ,l'gli's fdr'ceid inwardly permittinfVr the -llow of fluid from behind the seal forwardly past the seal. VBy this means the output of the piston portion 5S is added to the output of the piston portion 6@ prior to closure of the valve element 76. Y

The device of the present invention he designed to change the arrangement for pressurizing the braA 1 l ders at any desired pressure. Furthermore, any desired relative quantities of fluid may he displaced to the brake cylinders. The particular volumes and pressures siown herein are illustrative onlyi Purely by way of example in the illustrated embodiment, theY braise pressure :it the front brake-cylinders -2t? is 70 psi. at the time of closure of the valve element 76. Thisresults from the dille/rence in effective piston areas and the force of the spring MPS. While the pressure Iat ther'ear Wheel cylinders is l5() p.s.i. at the same time, this higher pressure is desirable to overcome the heavy return springs when using Duo Servo brakes on the rear wheels. cord'- ingly, the linings of front disc brakes and rear Duo Servo bralres will begin to apply braking load to their respective wheels at about the same time and in the desired amounts.

ln order to assist in the release of brake pressure from the front brake cylinders a pressure relier" passage ldd provides communication in the piston S6 from the charnbcr Mi to the passage 7d. This is normally closed by a ball check valve E42 held against a valve seal in the pasf sage lid@ a spring ldd. Whenever the pressure in chamber dit is greater than the pressure in the passage 7i?, the check valve 142 will open to bleed oli duid.

While it is apparent that the preferred embodiment illustrated herein is well calculated to fulfill the objects above stated, it` will be appreciated that the invention .is susceptible to change, modification and variation without departing Arom the fair meaning or proper scope of the subjoined claims;

What is claimed is:

l. In an hydraulic brake system for a vehicle having a master cylinder, a pair of front wheel brake cylinders, and a pair of rear wheel brake cylinders, a pressure transmitting device connected between the master cylinder and the brake cylinders, said device including a piston means having first, second and third piston portions, said iirst piston portion being arranged to disolace fluid toV the rear brake cylinders, said second and said third piston a, :leones 2. The structure set forth in clairnll in w f piston means operatively disposed in a housing having interconnected coaxial bores receiving said second and said third piston portions, respectively', and in which saidl second piston portion is provided with a unidirectonally actng seal engageable with the wall ofits associated bore, said seal being operable to permit the flow of Fluid thereby when the fluid being displaced by said third pis-tori portion is of a kgreater pressure than the fluid being displaced by said second piston portion. n

3. The structure set forth in claim l in which said last named means includes a pressure relief valve and a merober engageable with said pressurerelief valve for biasing said pressure relief valve in an open direction with a force proportional tothe differential pressure between the duid displaced by said second piston portion Vand the ilu-rd displaced byV said third pist/ontportion. f

4. ln a hydraulic brake system forv a vehicle y having a master cylinder, front wheel brake cylinders and rear Wheel brake cylinders, a pressure transmitting device con'- nected between the master cylinder and the brake cylinders; said device including a housing having lirst, second, third and four'thninterconnected coaxial'bores, a piston asse ably having irst, second, third and fourth piston portions operatively disposed iri said first, second, third fourth bores, respectively, said fourth piston portion being provided ,with a bore therein, a valve operating piston operativelydisposed in the bore of said fourth p portion, bypass means through said iirst, second, te. and fourth piston portions and o rough said valve op kating piston, a valve element operatively associated with said valve operating pistorna spring biasing said valve operating piston in a direction away troni ,said valve element, s"`id valve operating piston having an effective area exposed to master cylinder pressure during in braking which .tends to move said valve operating pi in a direction in which it closes with said valve to close Vsaid bypass means, said housing having a vent passage for said third bore, a spring biased pressure relief valve for closing said vent passage, a plunger exposed to the duid pressure `in said second and'third bores at the opposito ends thereof and engageable with said pressure relief valve td hiaspsaid pressureV relief valve invan open direction with a force proportional to the dierence in pressures between said second and third bores.

5. In an hydraulic brake system having a pair of front wheel brake cylinders and a pair of rear wheel brak cylinders, a pressure transmitting device operable to pres4 surize said frontend rear wheel. Vbrake cylinders, said device including aVK piston having integrally connected first, second, and third portions,`-said first portion being arranged to displace uid to the rear brake cylinders, said second and third portions being arranged to displace huid to the front brake cylinders, a passage tor the dow of fluid pressurized by said third piston portion to the rear brake cylinders when said iluid istat a pressure greater than the pressure of duid in said rear brake cylinders, and means for discontinuing the displacement of duid to the front bralre cylinders by said third piston portion after a selected initial brake application whereby the front brake cylinders will be ypresstu'ized solely byV said second piston portion during braking subsequent to said selected brake application.

6r In an Vhydraulic brake system for a vehicle having a pair of front brake cylinders and a pair of rear brake cylinders, a pressure transmitting device for pressurizing said brake-cylinders, said device including a pistonhaving integrally connected first, second and third portions of varying diameters, said first` piston portion being arranged to displace nid to the rearbralre cylinders, said second and third piston portions being arranged to displace duid to the front brake cylinders, and means for discontinuing the displacement of luid to the iront brake cylinders by said third piston portion after a selected initial bralre application 'whereby said frontV brake cylinders said brake cylinders, said device including a piston having integrally connected irst, second and third portions of varying diameters, said rst piston portion being arranged to displace fluid to the rear brake cylinders, said second and third piston portions ybeing arranged to displace fluid to thefront brake cylinders, means for discontinuing the displacement of uid to the front brake cylinders by said third piston portion after a selected initial brake application whereby said front brake cylinders will be pressurized v solely by said second piston portion after said selected brake application, and means for relieving the pressure of fluid displaced by said third piston portion after said selected initial brake application in accordance with the pressurization of the front brakes by said second piston portion.

References Cited by the Exannner UNITED STATES PATENTS 2,835,271 5/58 Uberthur 18S-152 r5.?091018 7/63 Stelzer 303-6 G. BOTZ, Primary Examiner.

FERGUS S; MDDLETON, Examiner. 

6. IN AN HYDRAULIC BRAKE SYSTEM FOR A VEHICLE HAVING A PAIR OF FRONT BRAKE CYLINDERS AND A PAIR OF REAR BRAKE CYLINDERS, A PRESSURE TRANSMITTING DEVICE FOR PRESSURIZING SAID BRAKE CYLINDERS, SAID DEVICE INCLUDING A PISTON HAVING INTEGRALLY CONNECTED FIRST SECOND AND THIRD PORTIONS OF VARYING DIAMETERS, SAID FIRST PORTION BEING ARRANGED TO DISPLACE FLUID TO THE REAR BRAKE CYLINDERS, SAID SECOND AND THIRD PISTON PORTIONS BEING ARRANGED TO DISPLACE FLUID TO THE FRONT BRAKE CYLINDERS, AND MEANS FOR DISCONTINUING THE DISPLACEMENT OF FLUID TO THE FRONT BRAKE CYLINDERS BY SAID THIRD PISTON PORTION AFTER A SELECTED INITIAL BRAKE APPLICATION WHEREBY SAID FRONT BRAKE CYLINDERS WILL BE PRESSURIZED SOLELY BY SAID SECOND PISTON PORTION AFTER SAID SELECTED BRAKE APPLICATION. 